In bubble memory devices, a serially ordered sequence of single wall magnetic domain positions represents bits of binary data. The distinction between binary "0's" and "1's" at a particular bit position in the sequence is represented by the corresponding presence or absence of a single wall domain (called a bubble).
Magnetic bubble technology provides a non-volatile, reliable, rugged, storage medium with high storage capability. A typical bubble memory device is fabricated by depositing an epitaxial thin film of magnetic material on a non-magnetic garnet wafer, and then forming lithographically patterned layers of a non-magnetic conductor (such as aluminum-copper alloy) and a high-permeability, low coercivity material (such as permalloy), with suitable insulation therebetween. The non-magnetic conductor is normally located between the thin film and the high-permeability layer. The device is packaged with permanent magnets that provide a static magnetic field (the bias field) oriented perpendicular to the film and coils that provide a rotating magnetic field in the plane of the film. A typical rotation rate is about 50-100 kHz.
The magnetic thin film has a crystallographic orientation such that small cylindrical magnetic domains (bubbles) are created by the action of the bias field. The bubbles are magnetized oppositely to the bias field while the much larger domain surrounding the bubbles is magnetized in the same direction as the field. The bubbles may be made to circulate by energizing the coils for the rotating field. The lithographically patterned layer of high-permeability material controls the movement of the bubbles in a precise fashion. The high-permeability material is deposited in a pattern of small chevrons or the like, referred to as propagators, to define the bubble paths.
A typical bubble memory architecture comprises a number of storage loops, an input track, and an output track. A bubble generator is coupled to the input track and injects bubbles that propagate to be exchanged into the loops to effect writing. A bubble detector is coupled to the output track and detects bubbles that are exchanged between the loops and the output track to effect reading.
Commercial magnetic bubble memory devices have used one of two techniques to generate bubbles, namely nucleation and replication.
FIG. 1 is a plan schematic illustrating a nucleating generator 10 according to known practice. Nucleating generator 10 comprises a U-shaped or hairpin conductor 12 (referred to as "hairpin 12") having a loop portion 13 overlapping a first propagator 15a at the beginning of a propagation track that comprises a plurality of propagators 15a, 15b, etc. In the illustration, the bias field, designated H.sub.b, is directed into the page and the rotating in-plane field, designated H.sub.i, is shown as a vector rotating clockwise. In operation, a nucleation current pulse, designated I.sub.n, is injected into hairpin 12 in a direction such that the magnetic field at the center of the loop is attractive for the bubble domain, that is, locally opposing the bias field H.sub.b at the center of the loop. The current pulse is injected into hairpin 12 at the time when the in-plane field H.sub.i is directed along the hairpin axis between the hairpin conductors, in a direction toward the loop portion (as indicated by the vector direction in the figure). A bubble 17 is nucleated at the center of the loop if the pulsed nucleation current is of sufficient magnitude. Although the required current magnitude depends on various device parameters and temperatures, a 200 milliamp, 200 nanosecond pulse is typical for room temperature operation. The nucleated bubble then proceeds down the propagation track as the in-plane field H.sub.i rotates.
FIG. 2 is a plan schematic of a replicating generator 20 according to known practice. Replicating generator 20 comprises a hairpin 22, a high permeability seed patch 23, a plurality of high permeability propagators 25, and a high permeability bar 26. Hairpin 22 is disposed with its loop portion 27 overlapping seed patch 23 and the hairpin conductors extending toward and crossing over the line of propagators 25.
The basic operation of replicating generator 20 is to cut or replicate a preexisting seed bubble that is present at seed patch 23 and then to transfer the cutoff portion to propagators 25. In operation, a replication current pulse, designated I.sub.r, is injected into hairpin 22 in a direction such that the magnetic field due to the current reinforces the bias field H.sub.b at the center of loop portion 27. The current is injected at the time that the in-plane field is passing through the phase indicated (as with nucleating generator 10). The seed bubble is stretched across the hairpin, the bubble in such stretched configuration being designated 28. The seed bubble is cut into two separate bubbles by the pulsed current I.sub.r if the current is sufficient to flip the magnetization at the hairpin center to create new domain walls. The required current magnitude depends on device parameters, but is typically 100 milliamps for 200 nanoseconds, and is largely temperature independent. The current through the hairpin is then reduced (typically to about 30 milliamps) for 90.degree. of field rotation. This pins the trailing part of the replicated bubble along the right outside edge of the hairpin so that as the in-plane field rotates 90.degree., the trailing part of the replicated bubble transfers to bar 26. As the field rotates another 90.degree. (pointing up in the figure), the bubble transfers to the propagation track. The leading portion of the replicated bubble remains on the seed patch so that it can be replicated again.
The known practice in the magnetic bubble field is to use one of these two types of generators. Each of the two bubble generation techniques has advantages and disadvantages, with the advantages of one representing the disadvantages of the other. This latter fact is not lost on the manufacturers of commercially available bubble memories, who are wont to tout the advantages of the type of generator present in their products, and are careful to point out the disadvantages of the other type.
The nucleating generator suffers from the disadvantage that its current must be temperature compensated. That is, extra circuitry must be provided to vary the nucleation current amplitude as a function of temperature to insure reliable operation over the temperature range of the device. Also, the nucleating generator is the most likely portion of the bubble memory to suffer a wear-out failure. This is because it is pulsed at a high repetition rate (once for every bubble that needs to be generated) and requires a current density in the hairpin of about twice that of a replicating generator.
On the other hand, the replicating generator must incorporate a mechanism for creating the seed bubbles (the discussion above regarding the replicating generator assumes that a seed bubble exists). This can be accomplished by operating the replicating generator in a nucleating generator mode by reversing the current polarity relative to that used for replication, but this requires additional control circuitry or dual-polarity power supplies in the system. As a result, loss of the seed bubble after the device is installed in an end user environment requires special maintenance procedures to restore functionality.
Depending on the particular device and system requirements, the particular disadvantages of one type of generator may be less of a problem than those of the other. In any event, the practice is to select the type of generator whose disadvantages are likely to be less disruptive for the particular application. To the extent that there is a significant reason to prefer one or the other, the non-chosen type of generator would be especially unsuitable.